Object-Oriented Programming in Java

public class Account {
    private final String owner;
    private double balance;

    public Account(String owner, double balance) {
        this.owner = owner;
        this.balance = balance;
    }

    public void deposit(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("amount must be positive");
        balance += amount;
    }

    public double getBalance() { return balance; }

    @Override
    public String toString() { return owner + ": " + balance; }
}

abstract class Shape {
    abstract double area();
}

class Rectangle extends Shape {
    private final double w, h;
    Rectangle(double w, double h) { this.w = w; this.h = h; }
    @Override double area() { return w * h; }
}

class Circle extends Shape {
    private final double r;
    Circle(double r) { this.r = r; }
    @Override double area() { return Math.PI * r * r; }
}

Shape[] shapes = { new Rectangle(2, 3), new Circle(1) };
for (Shape s : shapes) System.out.println(s.area());   // dynamic dispatch

interface Payable {
    double amountDue();
    default String label() { return "Payable"; }   // default method
}